Various cryogenic processes have been used in the past to recover ethane and heavier hydrocarbons from multicomponent gas streams such as natural gas, refinery gas and synthetic gas streams, which comprise mostly methane. A typical gas stream might contain about 90 wt % methane; about 5 wt % ethane; ethylene and other C.sub.2 components; and about 5 wt % heavier hydrocarbons such as propane, propylene, butanes, pentanes, etc. and non-hydrocarbon components such as nitrogen, carbon dioxide and sulfides. In a cryogenic process for ethane recovery, such a feed gas would be cooled and condensed to form a two-phase that would be separated. The vapor portion would be expanded in a turboexpander to a lower pressure, and one or more of the components would be fractionated in a demethanizer column to recover ethane. Residual gas leaving the demethanizer column would be compressed to feed gas pressure.
Ongoing efforts have been made to improve such processes, for example, by attempting to increase ethane recovery while reducing the external energy consumption. Accordingly, the present invention offers an improved cryogenic process having certain advantages, some of which are discussed specifically below.